There is a denial of service vulnerability in the If-Match and If-None-Match header parsing component of Sinatra, if the etag method is used when constructing the response and you are using Ruby < 3.2.
Versions of the package sinatra from 0.0.0 are vulnerable to Reliance on Untrusted Inputs in a Security Decision via the X-Forwarded-Host (XFH) header. When making a request to a method with redirect applied, it is possible to trigger an Open Redirect Attack by inserting an arbitrary address into this header. If used for caching purposes, such as with servers like Nginx, or as a reverse proxy, without handling the X-Forwarded-Host …
An issue was discovered in Sinatra 2.0 before 2.2.3 and 3.0 before 3.0.4. An application is vulnerable to a reflected file download (RFD) attack that sets the Content-Disposition header of a response when the filename is derived from user-supplied input.
Sinatra before 2.2.0 does not validate that the expanded path matches public_dir when serving static files.
Sinatra has XSS via the Bad Request page that occurs upon a params parser exception.
An issue was discovered in rack-protection/lib/rack/protection/path_traversal.rb in Sinatra on Windows. Path traversal is possible via backslash characters.